A new understanding of how plants and crops defend themselves against diseases could help to breed more successful disease-resistant agricultural crops, say researchers.

Despite efforts to control them, crop diseases still account for around 15% of the losses in the world’s food production. But, by exploiting new molecular and genetic insights, researchers may now be able to provide a better understanding of the defence system used by crops against the damaging pathogens that grow in the spaces between plant cells.

Writing in Trends in Plant Science, researchers from the UK and Netherlands suggest that there is now an ‘unprecedented’ opportunity to exploit new genomic information about crops and pathogens and use new knowledge about the operation of host defence against pathogen attack in plant breeding to produce crops with more durable resistance against damaging pathogens.

“As traditional methods of controlling crop disease become less effective, the need to breed new strains of crops with an inbuilt resistance to the disease pathogens increases,” explained first author, Dr Henrik Stotz from the University of Hertfordshire.

“In the same way that humans have developed immune responses against human disease pathogens, crops can be bred for resistance against disease pathogens, but we need to improve our understanding of effective resistance mechanisms within plants,” he said.

“Our research enhances the traditional understanding of the plant defence system and describes a new concept describing how plants protect themselves against the pathogens that grow in the space outside plant cells (the apoplast) – a new concept called effector-triggered defence or ETD.”

The ETD concept could provide opportunities to improve the effectiveness of breeding crops for resistance against disease, and increase food security, said the team.

“This new understanding of plant defence through ETD suggests different operations of specific resistance genes which will help us to be more successful in breeding new strains of crops for resistance,” added senior author Professor Bruce Fitt – also from the University of Hertfordshire.

“This is essential in the battle for global food security to protect the world’s future food sources.”

Plant defence

The team explained that a plant’s defence system is made up of interconnected tiers of receptors, which are found both outside and inside the plant cells. Both sets of receptors sense the invasive pathogen and respond to its intrusion.

The current understanding of plant defence is that plants, using these receptors, have two forms of defence. Pattern-triggered immunity (PTI) is the first line of defence, operating soon after the pathogen has landed on the plant surface. The second line of defence is referred to as effector-triggered immunity (ETI), and is based on the detection of disease pathogens by the plant’s genes.

“This concept of plant ETI does not really explain the second line of defence in the interaction of plant hosts protecting themselves against extracellular fungal pathogens – i.e. those foliar fungal pathogens that get into the leaf of the plant to exploit the space between its cells, known as the apoplast, to retrieve nutrients from the plant,” explained Stotz. “These include the damaging pathogens that cause septoria leaf blotch on wheat, barley leaf blotch, apple scab and light leaf spot on oilseed rape.”

“The ETI concept does not hold for defence against those pathogens that go into the leaf but not into the cells. “

New knowledge

Indeed, Stotz and his colleagues discovered that the genes involved in defence against extracellular pathogens (ETD) are different from those involved in the defence against intracellular pathogens.

“We identified some specific resistance genes that code for receptor-like proteis (RLPs) and described how they operated against the pathogens,” he explained. For example, the team noted that if resistance against all apoplastic pathogens is mediated by genes encoding RLPs, then for crops attacked by apoplastic pathogens “it should be possible to screen their genomes, especially those regions identified as containing loci for resistance against these pathogens, specifically for R genes that encode RLPs.”

“The genes identified can then be considered as candidate ETD resistance genes,” the team wrote. “If effective R genes operating against apoplastic pathogens and the virulence spectra of their attacking pathogens can be identified more rapidly and deployed more carefully, they will make a substantial contribution to sustainable crop protection and improved food security.”